This project deals with phenomena characterizing a house mouse population as it proceeds through successive stages of crowding resulting from several population doublings in an eight cell, experimentally-designed habitat persumed optimum for 100 mice. From the initial 16 colonizers the population has been permitted to increase each 200 days from 16 to 100 to 200 to 400 to 800 mice. This year's report concentrates on the establishment of adult social groups during the 60 days out of each 200 days when progeny were permitted to remain in the population to form the next generation. The design of the habitat represents a major aspect of this study. It is an octagon, 16 feet across, with metal walls. Major resources such as nesting boxes, food and water are located on the walls, the mice gain access to them by climbing vertical wire ramps. Low radial barriers, over which the mice can climb, divide the habitat into 8 pie-shaped cells. Lower barriers divide each cell into 2 subcells such that the universe may be considered to consist of 16 subcells. The counter clockwise subcells are apparently preferred because their nest boxes are one-half the height of those in clockwise subcells, and less energy expenditure is required to move between the nest boxes and other resources.